1. Field of Invention
This invention relates to sensitizers for photopolymerizable compositions. More particularly, this invention relates to high speed photopolymerizable printing plate compositions of ethylenically unsaturated monomers and oligomers by the use of merocyanine sensitizers in combination with photoinitiators such as iodonium salts and trichloromethyl-s-triazines.
2. Background of the Art
Photopolymerizable compositions have been used in photosensitive elements, such as printing plates, for a number of years. Most representative of the prior art are compositions disclosed in U.S. Pat. Nos. 3,218,167 and 3,887,450. These compositions are relatively slow, however, and must be exposed in the absence of oxygen. Radiation sensitive elements which are oxygen insensitive have been produced by overcoating the elements with an oxygen barrier layer as in U.S. Pat. No. 3,895,949. These elements nevertheless remain relatively slow and such compositions have been satisfactory only where contact printing of the plate and relatively long time exposures can be tolerated.
Radiation sensitive systems capable of initiating free radical polymerization upon absorption of radiation are well known in the art, such as Free-Radical Chemistry, D. C. Nonhebel and J. C. Walton, University Press (1974). Suitable free radical generators can be selected from many classes of organic compounds including, for example, organic peroxides, azo compounds, aromatic diazonium salts, aromatic iodonium salts, aromatic sulfonium salts, aromatic phosphonium salts, quinone, benzophenones, nitroso compounds, acyl halides, aryl halides, hydrazones, mercaptocompounds, pyrylium compounds, triarylimidazoles, biimidazoles, haloalkyltriazines, etc. These materials, generally, must have photosensitizers.
Additional reference in the art to free radical photoinitiator system for ethlyenically unsaturated compounds are included in U.S. Pat. No. 3,887,450 (e.g.,column 4), U.S. Pat. No. 3,895,949 (e.g., column 7),and U.S. Pat. No. 4,043,810. Preferred initiators are the onium salts as disclosed in U.S. Pat. Nos. 3,729,313, 4,058,400 and 4,058,401. Other desirable initiators are halotriazines as disclosed in U.S. Pat. No. 3,775,113 and Biimidazoles disclosed in U.S. patent application Ser. No. 824,733, filed Aug. 15, 1979 and U.S. Pat. No. 4,311,783. These references also disclose sensitizers therein. A further reference to photoinitiator systems is "Light Sensitive Systems," J. Kosar, 1965, J. Wiley and Sons, Inc. especially Chapter 5.
Photopolymerizable compositions with higher sensitivity are required for applications such as projection printing plates and laser addressable printing plates. For these applications and particularly for laser addressable printing plates, high speed is essential to keep the exposure time short. It is also important to be able to use low intensity lasers, which are low cost and more reliable than the high intensity lasers. For these reasons, various studies are found in the art directed towards improving the sensitivity of photopolymerizable compositions. Some of the representative patents which disclose high speed photopolymerizable compositions are U.S. Pat. No. 4,228,232 (3M); U.S. Pat. No. 4,594,310 (Mitsubishi Chem Ind KK); U.S. Pat. No. 4,259,432 (Fuji Photo Film); U.S. Pat. Nos. 4,162,162, 3,871,885 and 4,555,473 (Dupont); U.S. Pat. No. 4,147,552 (Eastman Kodak) and European Patent Nos. 109,291 (Fuji Photo Film) and 196,561 (Nippon Oils and Fats) and Belgium Patent No. 897,694 (Nippon Oils and Fats).
Although some of these photopolymerizable compositions exhibit high sensitivity (&lt;1.0 mj/cm.sup.2) in the UV region, they have lower sensitivity (&gt;2.0 mj/cm.sup.2) at visible wavelengths. The lower sensitivity in the visible region of the spectrum restricts the utility of these photopolymerizable compositions. For example, commercially available low cost lasers such as argon ion lasers (water-cooled and air-cooled) have strong light output at 488 nm and 514 nm which the UV sensitive materials cannot properly utilize.
This invention shows how highly sensitive aqueous developable photopolymerizable printing plate compositions which can be imaged by visible light projection exposure or by scanning with low powered visible light emitting lasers (such as argon-ion lasers) can be made by employing very efficiently sensitized free radical initiator systems, together with highly reactive free radically polymerizable materials (monomers and oligomers) and oxygen barrier layers.
U.S. Pat. No. 4,228,232 (3M) describes aqueous developable photopolymerizable compositions containing unsaturated oligomers for applications such as printing plates. The initiators for these compositions are sensitized iodonium salt and are sensitive to visible light. These compositions can be imaged by exposing with a water cooled argon ion laser (488 nm) and a photo sensitivity of up to 1.8 mj/cm.sup.2 for 5 solid steps, or sensitivity of 0.32 mj/cm.sup.2 for 1 solid step ( 2 density, step wedge) was obtained.
European patent No. 195,561 describes a photosensitive lithographic printing plate which can be imaged by Argon-Ion Laser. The composition contains a polymerizable compound, an organic peroxide and a compound absorbing visible light. In a similar patent (BE 897,696), a photopolymer composition with sensitivity threshold of at least 480 ergs/cm.sup.2 at a wavelength of 488 nm is described.
Compounds capable of generating free radicals upon exposure to light, which initiates polymerization, are well known in the art ("Photopolymerization of Surface Coatings", C. G. Roffey, Wiley-Interscience Publications, 1982, pages 67-136). However, the utility of many of these photoinitiators or photoinitiator systems are quite limited because the initiators are activatible primarily in the ultraviolet region of the spectrum. For example, iodonium salt photoinitiators are known to absorb ultraviolet light only at wavelengths below about 300 nm, and trichloromethyl-s-triazines absorbs light below 350 nm.
Various investigations have been carried out in order to broaden the spectral response of photoinitiators. To achieve this, researchers have added compounds which absorb incident light more efficiently and then activate the photoinitiator to produce free radicals. These compounds which absorb the incident light more efficiently are generally termed as sensitizers or photosensitizers.
U.S. Pat. No. 3,729,313 describes sensitizers for diaryliodonium compounds for free radical polymerization. Sensitizers are included from the class of diphenylmethane, xanthane, acridine, methine and polymethine, thiazole, thiazine, azine, aminoketone, porphyrin, colored aromatic polycyclic hydrocarbons, p-substituted aminostyryl compounds and aminotriaryl methanes.
Japanese patent J6 0088005 (Agency of Ind. Sci. Tech.) discloses 3-ketocoumarin compounds as sensitizers for diaryliodonium compounds.
U.S. Pat. No. 4,250,053 describes sensitizers for Iodonium salts and particularly 1,3-diaryl-1-pyrazolines for free radical polymerization.
U.S. Pat. No. 3,617,288 describes sensitizers for halogenated hydrocarbon compounds.
U.S. Pat. No. 4,505,793 describes ketocoumarin sensitizers for halogenated triazines for free radical polymerization.
While the aforementioned compositions have provided improved photospeed compared to the unsensitized system, there is still a need for a more sensitive photopolymer composition. High photospeed is particularly desirable for projection exposure imaging and imaging by laser scanning techniques.
U.S. Pat. Nos. 4,162,162, 4,268,667, 4,351,893 and European Patent No. 127,762 disclose sensitizers containing constrained amino-ketone groups for bi-imidazole initiators in addition polymerization. U.S. Pat. No. 4,505,793 discloses constrained coumarin sensitizers for triazine initiators. Amino group containing coumarins and constrained coumarins are also disclosed in U.S. Pat. Nos. 4,278,751, 4,147,552, 4,366,228 and G.B. Patent No. 2,083,832 for use as triplet sensitizers for cyclo-addition reactions or for use as sensitizers for radical polymerization in combination with arylaminoacetic acids. References of these compounds are also found in Polym. Eng. Sci. 1983, 23, 1022-1024. This article mentions the utility of aminoketocoumarin compounds as sensitizers with alkoxypyridinium salts. Photoreactions of coumarin compounds are also mentioned in J. Org. Chem. 1984, 49, 2705-2708. U.S. Pat. No. 4,250,053 teaches constrained coumarin as a sensitizer of iodonium salt for cationic polymerization.